In space-based crosslinks, microwave communication links must be highly efficient on both the transmit and the receive end of the link. This is a different problem from traditional satellite-to-ground communication links. In these traditional communication systems, the ground-based side of the link is not limited in terms of antenna size, weight and power consumption. The traditional systems, therefore, have the freedom to design microwave communication links that provide high efficiency, small size and low weight on the space-based end of the link while making up for the performance shortcomings on the ground side of the link. For example, spacecraft antennas are kept small to minimize weight, while ground antennas are usually large with very high antenna gain. Also, microwave power amplifiers on board spacecraft are usually minimized since additional antenna gain on the ground can compensate for lower transmit power. In the same way, additional transmit power from the ground is much easier to generate since it does not need to be highly efficient. In satellite-to-satellite crosslink communication systems, both ends of the link are space-based. Therefore, the overall direct current (DC) power efficiency of the link must be maximized.
In a space-based microwave crosslink system, the major microwave components which comprise the link and thus define overall link efficiency are the transmit and receive antennas, the transmit power amplifier and the receiver low noise amplifier. In order to improve link performance without impacting power consumption, antenna gain can be increased by providing a larger aperture antenna. However, since both ends of the link are space-based, there is a practical limit to the maximum antenna size due to typical satellite size and weight constraints. Also, higher antenna gain requires better pointing accuracy and more stable platforms in order for the link to acquire. Since both ends of a crosslink are mobile, this further limits maximum antenna gain. With the above fundamental limitations, there are no straight-forward improvements to antenna gain that can solve the overall efficiency problem.
Incremental improvements in device technology, circuit topology, and communication system implementation have improved microwave power amplifier efficiency. Even with these improvements, power amplifiers remain the largest power consumption component in microwave communication systems. As a result, power amplifiers are the focus of most efficiency improvement effort. The primary objective is, in general, to provide the required radio frequency (RF) output power with the least amount of DC power output.
In order to provide a crosslink system with minimum RF output power from the power amplifier, the low noise amplifier (LNA) must be improved. One method of improving noise figure is by utilizing better device technology such as InP devices or very short gate lengths (&lt;0.1 .mu.m). However, this technology is very expensive and provides only incremental improvement in noise figure performance. Another way of improving noise figure is by cooling the LNA receiver. It is known that noise figure for a given LNA improves at a rate of approximately 0.015 dB per degree Celsius as the operating temperature of the amplifier is reduced. However, most cooling systems are large, heavy and unsuitable for use in space-based communication systems.
Therefore, what is needed is a highly efficient space-based microwave crosslink communication system in which both ends of the crosslink are optimized in terms of size, weight and power consumption. What is also needed is an efficient method of cooling an LNA suitable for use in a space-based crosslink communication system. Further needed is an integrated packaging approach which allows efficient localized cooling of the LNA for improved performance without requiring excessive cooling power .